Aeration of plaster of paris compositions



Patented May 11, 1943 AER-ATION OF PLASTER OF PARIS COMPOSITIONS companyConrad Gerard Francois Cavadino, Rochester, England, assignor to GyprocProducts Limited, London, England, a British limited-liability NoDrawing. Application December 19, 1939, Se-

rial No. 309,970. In Great Britain December 1 Claim.

The present invention relates to the aeration of plaster of Pariscompositions, particularly for the manufacture of articles such asplaster boards.

In the manufacture of plaster boards of the kind in which the plasterforms the core of the board andhas a paper covering constituted by papersheets known as liners, a bond is formed between the core and the paper,the bond being due to the strong needle-shaped crystals of hydratedgypsum CaSO4,2H2O, which are formed in the core during rehydration ofthe plaster, and which attach themselves to the liners. In the processof manufacturing these boards, the dry plaster is generally spread uon abelt, and soaked in water by drawing the belt slowly through awater-bath to ensure that the plaster acquires a workable consistency.After this operation, a plaster of Paris from one source will be in thestate of a quite light liquid slurry, while plaster from another sourcewill have a more muddy and heavier consistency, and if it is required toform porous boards of the same density from two such different plastersthe aeration of the plaster having the more muddy and heavierconsistency will require the evolution of a greater volume of aeratinggas, and a higher gas pressure, than will the aeration of the otherplaster.

The aeration of plaster of Paris compositions for making plaster boards,by oxygen gas set free as a result of a chemical reaction has alreadybeen proposed; in particular, it has been proposed for this purpose toliberate oxygen by means of the reaction of an oxidising agent withbarium peroxide in the presence of hydrochloric acid or nitric acid orboth.

' It has now been found that in the application of this known process tothe manufacture of plaster boards from certain kinds of plaster ofParis, the use of mineral acids presents certain disadvantages. Thechief of these disadvantages isthat with a plaster of Paris, which givesa slurry of a muddy and heavy consistency and the aeration of whichcalls for a high concentration of reagents in order that the necessarygas pressure" and aeration volume may be produced, the presence of amineral acid in the necessary high concentration causes the plaster ofParis to be deleteriously attacked. In the manufacture of plaster boardswhich have a covering constiprises the use of a ceti c acid, in place ofthe mineral acids previously employed.

Acetic acid has the advantage over mineral acids that it may be used atany concentration without deleteriously affecting the plaster core ofplaster boards, when high concentrations of acid are necessary byreasons of the nature of the plaster and of the slurry made therefrom.

According to the present invention the process for producing an aeratedplaster of Paris composition comprises the steps of forming a slurry oflaster of Paris and water and producing bub- Eles of oxygen in saidslurry by the reaction of potassium permanganate with barium peroxide mt e p e o ace 1c acid.

In carrying out the invention, there is added to the slurry of plasterof Paris and water, a solution of potassium permanganate, and there isalso added to the slurry barium peroxide dis solved in acetic acid. Thetwo solutions must of course be added to the slurry from differentsources, and preferably only mingle in the slurry, But they may be addedto them ly and preferably one behind the other on the belt which carriesthe slurry to the mixer.

The barium peroxide in the presence of acetic acid reacts with thepotassium permanganate to evolve oxygen gas.

ermca reaction in the processes of this inventiTm has the advantage thatit does not occur immediately the reagent substances come together, buttakes placeaf tera retardation of a ,few seconds; this enables a gooddispersion of the reagent substances throughout the slurry to beproduced by a rapid stirring. Thereafter the reaction goes rapidly tocompletion with the production of a very large number of minute oxygenbubbles, the final aerated product being thereby further advantageouslycharacterized by a great number of small voids and being therefore ofgreater strength than if the voids were larger and fewer in number. Withthe present invention the use of soaps, gums or resins is quiteunnecessary and findesiiable, because although these substances tend toensure the production of small bubbles, they have the disadvantage thatthey retard the completion of the chemical reaction to such an extentthat in the manufacture of plaster board for example, the reaction isnot completed before the plaster enters the liners, and in suchmanufacture it is essential that this reaction should have reachedcompletion before the plaster enters the liners; moreover suchsubstances as soap, gum and resin, in the presence of an acid,considerably decrease the volume of aeration to an extent which maycorrespond to as much as sixty per cent.

In the preferred method of carrying the invention into effect, theaeration reagents are made up into two solutions as previously statedwhich are added to the slurry of plaster of laris either as the slurryis travelling on the soak belt, or while travelling on a cross belt ontowhich the soak belt discharges. In either case the addition is made tothe slurry shortly before it reaches the mixing vessel into which it isdischarged and wherein it is vigorously stirred and from which itemerges in the case of plaster board manufacture with liners, to bespread on to one liner and then covered by another.

The advantages gained by the use of the present invention will be stillfurther made clear by the example given below, in which are given'thequantities necessary for the aeration of a slurry 22 about 6,800 lbs.plaster of Paris.

Example Solution (a) Water gallons About 15 Potassium permanganateounces 24 Solution (b) a er ga110ns About 35 Acetic acid (concentratedto 99% by weight) gallons 0.65 Barium peroxide lbs 5 The reactionbetween these two solutions engenders a gas pressure of about 60 mm. ofmercury.

The results of the reaction are as follows:

Reaction Pressure in periods A mm mm. in mercury tilon. v0 ume g gg 23;?in c. c. Freshly After in sees. in secs. made 20 The figure of aerationvolume given above is the result of the reaction between 100 c. c. ofsolution a and 250 c. c. of solution b when mixed togamer, without theslurry, in an open vessel, the volume of the resultant foam producedbeing measured; and those relating to pressure were obtained bymeasuring the gas pressure produced when 10 c. c. of solution a and 25c. c. of solution b reacted in a conical flask of 560 c. 0. volume,

10 The latter being connected to a mercury manometer, and such pressurewas found when the atmospheric pressure was 762 mm. and at a temperatureof 20 C.

The above example is given to illustrate the order of the proportions inwhich the various reagents may be employed, and it is to be understoodthat the compounding of the two solu tions can depart widely from theexamples, depending upon the nature of the plaster of Paris used, uponthe gas pressure desired, and upon the volume of gas which it is desiredto generate.

I claim:

A process for producing aerated plaster of Paris compositions andarticles made therefrom, comprising the steps of forming a slurry ofplaster of Paris and water and producing bubbles of oxygen in saidslurry by the reaction of potassium ence of acetic acid.

CONRAD GERARD moors CAVADINO.

